1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus, such as a SAW filter, and more particularly, to a SAW apparatus having a balanced-to-unbalanced conversion function and having different impedance characteristics at an input side and an output side of the SAW apparatus. The present invention also relates to a communications unit including the above-described SAW apparatus.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and weight of portable communications units, such as cellular telephones. One method to decrease the size and weight is to reduce the number and the size of the individual components of communications units. Additionally, components having composite functions are being developed.
In view of this background, as a SAW filter used in an RF stage, a balanced filter provided with a balanced-to-unbalanced conversion function (so-called xe2x80x9cbalun functionxe2x80x9d) or a multi-band filter provided with a plurality of pass bands is used.
For example, Japanese Unexamined Patent Application Publication No. 10-117123 discloses a balanced SAW filter unit which utilizes an unbalanced input and a balanced output by combining SAW filters whose transmission phase characteristics are 180xc2x0 out of phase with each other. The configuration of the SAW filter unit disclosed in the above publication is shown in FIG. 9.
The SAW filter unit shown in FIG. 9 includes longitudinally coupled SAW filters 511 and 512. The longitudinally coupled SAW filter 511 is formed by cascade-connecting SAW filter devices 501a and 501b in two stages via connecting patterns 520 and 521. The longitudinally coupled SAW filter 512 is formed by cascade-connecting a SAW filter device 501c and a SAW filter device 502, having transmission phase characteristics are 180xc2x0 out of phase with each other, in two stages via a connecting pattern 521 and a connecting pattern 522.
In the above-described SAW filter unit, the input terminals of the SAW filter devices 501a and 501c are respectively connected to an unbalanced terminal 503 via wires 530 and 531. The SAW filter devices 501b and 502 are connected in series with each other via a wire 505, and are also respectively connected to output balanced terminals 504 via wires 532 and 533. Terminals 506 are ground terminals, which are respectively connected to the SAW filters devices 501a and 501c via wires 534 and 535.
An example of the multi-band filter is disclosed in Japanese Unexamined Patent Application Publication No. 10-341135 in which a plurality of SAW filter devices (SAW filters) are arranged on a piezoelectric substrate to obtain a plurality of pass bands.
In the above-described SAW filters having composite SAW filter devices, the plurality of SAW filter devices are provided on the same piezoelectric substrate. In order to reduce the size of such SAW filters, two or more SAW filter devices are disposed side by side in a SAW propagating direction.
However, such an arrangement of SAW filter devices causes ripples in the pass band of a transmission signal due to an influence of bulk waves.
In RF filters used in cellular telephones, the transmission characteristics in the pass band should be flat. The generation of ripples must be prevented to produce flat transmission characteristics.
One method of preventing the generation of ripples is to scatter bulk waves by roughening the reverse surface of a piezoelectric substrate (forming undulations by sandblasting). According to this method, however, the piezoelectric substrate is often broken or warped due to heat or stress applied to a wafer during processing.
In the above-described known SAW filters, the generation of ripples in the pass band is prevented by displacing a plurality of SAW filter devices such that they are not disposed in the corresponding SAW propagating directions. Such an arrangement of the SAW filter devices, however, increases the size of the substrate. Accordingly, there is a demand for a more compact SAW filter (SAW apparatus) in which ripples caused by bulk waves are reduced without performing sandblasting.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a SAW apparatus including a first SAW filter device having a plurality of interdigital electrodes (interdigital transducers, which are hereinafter referred to as xe2x80x9cIDTsxe2x80x9d) provided on a piezoelectric substrate along a direction in which a SAW propagates, and a second SAW filter device including a plurality of IDTs provided on the piezoelectric substrate along a direction in which a surface acoustic wave propagates. The propagation path of the first SAW filter device partially overlaps the propagation path of the second SAW filter device. When the center-to-center distance between the first SAW filter device and the second SAW filter device is indicated by d1, and when the thickness of the piezoelectric substrate is indicated by t, the first SAW filter device, the second SAW filter device, and the piezoelectric substrate are arranged to satisfy the condition d1xe2x89xa62.3xc3x97t or d1xe2x89xa72.8xc3x97t.
With this arrangement, the above-described SAW apparatus is provided with, not only a filtering function having a specific pass band, but also composite multiple functions by providing a plurality of SAW filter devices.
In the above-described configuration, since the first SAW filter device, the second SAW filter device, and the piezoelectric substrate are arranged to satisfy the condition d1xe2x89xa62.3xc3x97t or d1xe2x89xa72.8xc3x97t, an influence of bulk waves caused by the excited SAW is effectively prevented between the first and second SAW filter devices.
Accordingly, ripples caused by bulk waves occurring in the transmission signal in the pass band are greatly reduced while achieving a reduced size and multiple functions of the SAW apparatus. Thus, flatter filtering characteristics are obtained such that the transmission characteristics in the pass band are greatly improved.
Additionally, according to the above-described configuration, an influence of bulk waves is prevented without performing sandblasting (to roughen the reverse surface of the piezoelectric substrate). Thus, breakage or warpage of the piezoelectric substrate caused by roughening the piezoelectric substrate is prevented, thereby decreasing the number of defective components produced in the manufacturing process.
In the aforementioned SAW apparatus, the transmission amplitude characteristics of the first SAW filter device in the pass band are preferably substantially the same as those of the second SAW filter device in the pass band. The transmission phase characteristics of the first SAW filter device are preferably substantially 180xc2x0 out of phase with those of the second SAW filter device. An input terminal or an output terminal is an unbalanced terminal, and the other terminals are balanced terminals.
With the above arrangement, since the transmission phase characteristic of the first SAW filter device is about 180xc2x0 out of phase with that of the second SAW filter device, a balun function is provided, thereby providing multiple functions for the SAW apparatus.
In the aforementioned SAW apparatus, the center frequency of the first SAW filter device is preferably different from that of the second SAW filter device such that the SAW apparatus functions as a multi-band filter. With this arrangement, the SAW apparatus is provided with composite multiple functions.
Another preferred embodiment of the present invention provides a SAW apparatus including first through fourth SAW filter devices, each having a plurality of IDTs provided on a piezoelectric substrate along a direction in which a SAW propagates. The first SAW filter device and the third SAW filter device are cascade-connected, and the second SAW filter device and the fourth SAW filter device are cascade-connected. The propagation path of the first SAW filter device partially overlaps with that of the second SAW filter device, and the propagation path of the third SAW filter device partially overlaps with that of the fourth SAW filter device. When the center-to-center distance between the first SAW filter device and the second SAW filter device is indicated by d1, when the center-to-center distance between the third SAW filter device and the fourth SAW filter device is indicated by d2, and when a wavelength of a SAW to be excited is indicated by xcex, the first through fourth SAW filter devices are arranged such that the center-to-center distance d1 is different from the center-to-center distance d2 by an amount equal to approximately (2n+1)xc3x970.5xcex, (n=0, 1, 2, 3, and so on).
With the above-described configuration, by providing the first through fourth SAW filter devices, a filtering function having a specific pass band is provided, and composite multiple functions are obtained.
In the above-described configuration, the center-to-center distance d1 is different from the center-to-center distance d2 by an amount that is substantially equal to (2n+1)xc3x970.5xcex (n=0, 1, 2, 3, and so on), i.e., a multiple of an odd number, of the SAW half-wavelength. Accordingly, even if the first and second SAW filter devices are located in close proximity to the third and fourth SAW filter devices to reduce the size of the SAW apparatus, ripples occurring between the first and second SAW filter devices and the third and fourth SAW filter devices caused by bulk waves are greatly reduced.
Thus, a filtering function is provided, and also, generation of ripples are reduced while achieving a smaller SAW apparatus provided with multiple functions. Accordingly, flatter filtering characteristics are obtained, thereby improving the transmission characteristics in the pass band.
In the aforementioned SAW apparatus, the transmission amplitude characteristics of the first through fourth SAW filter devices in the pass band are preferably substantially the same. Three of the first through fourth SAW filter devices preferably have substantially the same transmission phase characteristics, and the transmission phase characteristic of the remaining SAW filter device is preferably 180xc2x0 out of phase with that of the other SAW filter devices. One of an input terminal and output terminals may be an unbalanced terminal, and the other terminals may be balanced terminals.
With this arrangement, by setting one of the SAW filter devices out of phase with the other three SAW filter devices, a balun function can be provided, thereby achieving multiple functions.
In the aforementioned SAW apparatus, when the thickness of the piezoelectric substrate is indicated by t, the first through fourth SAW filter devices and the piezoelectric substrate may be arranged so as to satisfy conditions d1xe2x89xa62.3xc3x97t or d1xe2x89xa72.8xc3x97t and d2xe2x89xa62.3xc3x97t or d2xe2x89xa72.8xc3x97t.
With this arrangement, by setting the center-to-center distances d1 and d2 as described above, the generation of ripples is prevented, thereby improving the transmission characteristics.
In the aforementioned SAW apparatus, the piezoelectric substrate is preferably a 36xc2x0 to 44xc2x0 Y-cut X-propagating LiTaO3 substrate. The insertion loss is reduced by high piezoelectric characteristics, and an influence of the environmental temperature is suppressed by excellent temperature characteristics.
A communications unit according to preferred embodiments of the present invention utilize one of the aforementioned SAW apparatuses according to other preferred embodiments of the present invention.
With this configuration, since a compact and multi-function SAW apparatus having outstanding transmission characteristics is utilized, the size of the communications unit is greatly decreased while exhibiting an outstanding transmission/reception function.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.